The invention provides an improved drainage disc for use in composite membrane assemblies. This improved drainage disc increases the through-put of filtrate and overcomes problems associated with thermal cycling.
Membrane assemblies are used in a variety of liquid-solid separation processes. Membrane assemblies may be used either by themselves or in conjunction with additional membrane units for the retention of soluble marcromolecules or suspended colloidal particles. Soluble macromolecules are proteins, polypeptides, polysaccharides, oligosaccharides, polyphenolics, synthetic water soluble polymers, and the like. Examples of suspended colloidal particles are pigments, dispersed dyes, oil-in-water emulsion globules, fat emulsions, polymer latices and dispersions, metal/non-metals/oxides/salts, dirt, soils, clays, micro-organisms, and plant/animal cellular materials.
Membrane assemblies are typically in the form of rectangular plates consisting of a plastic backing plate with a membrane permanently attached to each side. The membrane is cast on a fine support scrim, and a layer of coarse textured, non-woven fabric such as, spun-bonded polyester or polyolefin, is placed between the scrim and backing plate to allow flow of permeate or filtrate to an extraction point with a minimum of resistance. The membrane is either an ultrafiltration, microporous, or reverse-osmosis membrane. The filtrate is collected and removed from each system. A typical ultrafiltration system is disclosed in Dorr-Oliver Inc., Bulletin Number 10-6, entitled "Series `S` Ultrafiltration System for Feasibility Studies-Economic Modeling-Process Development-Product Development-Primary Processing".
Early on it was recognized that drainage of the filtrate from the area between the membrane and backing plate was quite important to the application of membrane assemblies in the separation of large quantities of particle slurries.
In the past, the membrane was heat sealed to an impervious backing plate along its edges and about a center hole in the plate. The filtrate was evacuated through the drainage mat or scrim away from the center hole and out along the ends of the backing plate. This design is described in U.S. Pat. Nos. 4,264,447 (Nicolet) and 4,302,270 (Nicolet).
It was later determined that the application of a thin wafer around the center hole of the backing plate could provide ready drainage of the filtrate through the center hole and out of the membrane assembly. In accordance with the thin wafer design, four grooves were carved into the backing plate toward the center hole. Instead of heat sealing about the center hole as done in the Nicolet patents, a thin wafer of pure polyethylene or polypropylene was disposed on both sides of the backing plate. The thin wafers were donut shaped and heat sealed to membranes. The thin wafers were not attached to the backing plate, but were permitted to float as filtrate passed between the wafers and the backing plate into the center hole.
One problem with using a thin wafer is that the wafer itself is temperature sensitive and has a tendency to crack during thermal cycling, such as sterilization or when a heated filtrate is passed there-between. Also, due to the desired thinness of the wafer it is difficult to mold them using mineral-filled polymers, thus restricting application and design. It was also found to be labor consuming to heat seal wafers to each membrane of every membrane assembly.
The present inventor has developed a novel drainage disc for use in membrane assemblies which overcomes the disadvantages set forth above with regard to the prior art. In particular, it was discovered that the cracking of the thin wafer was a result of different expansion coefficients between the materials used to make the thin wafer and backing plate. The thin wafer is composed of pure polyethylene or polypropylene, whereas the backing plate was composed of mineral-filled polyethylene or polypropylene. To overcome this shortcoming, the present inventor has designed a new drainage disc formed of a material having a thermal expansion coefficient substantially the same as the material used in making the backing plate. By using materials having similar expansion coefficients, a drainage disc has been designed which avoids cracking during thermal cycling.
Also, an increase in filtrate through-put up to 20% has been observed when using the new drainage disc of the present invention. Additional advantages of the present invention shall become apparent as described below.